1. Field of the Invention
This invention pertains to visibility meters, such as are used in metereology, aeronautics and in automotive traffic-control systems as a warning device detecting the presence of fog.
2. Description of the Prior Art
Prior-art visibility meters have proved to be subject to malfunction as a result of weather conditions and environmental disturbances. In prior-art devices, maintenance is required in order to clean the exterior surfaces of the optical systems in the light transmitter and the light receiver. If, for example, dust particles land upon either the light transmitter or the light receiver, readings derived from the visibility meter will be inaccurate. This can lead to severe problems, especially in the event that the visibility meter is installed at, e.g., an airport where accurate visibility measurements are essential.
Therefore, it would be advantageous to provide a visibility meter which would require only minimal maintenance and which would be less sensitive to the influences of weather conditions and environmental disturbances, such as the presence of insects and the like.